Prior Art FIG. 1A illustrates a head and disk system 10, according to conventional art. In operation, a magnetic transducer 20 is supported by the suspension 13 as it flies above a disk 16. Magnetic transducer 20, usually called a “head” is composed of elements that perform the task of writing, with write head 23, magnetic transitions and reading, with read head 12, the magnetic transitions. The electrical signals to and from read head 12 and write head 23 travel along conductive paths (leads) 14 which are attached to or embedded in suspension 13. Magnetic transducer 20 is positioned over points at varying radial distances from the center of disk 16 to read and write circular tracks (not shown). Disk 16 is attached to a spindle 18 that is driven by a spindle motor 24 to rotate disk 16. Disk 16 comprises a substrate 26 on which a number of thin films 21 are deposited. Thin films 21 include ferromagnetic material in which the write head 23 records the magnetic transitions in which information is encoded.
Typically, conventional disk 16 includes substrate 26 of glass or AlMg with a coating of Ni3P that has been highly polished. The thin films 21 on disk 16 typically include an underlayer of chromium or chromium alloy and at least one ferromagnetic layer based on various alloys of cobalt. For example, a commonly used alloy is CoPtCr. Additional elements such as tantalum and boron are often used in the magnetic alloy. A protective overcoat layer is used to improve wearability and corrosion resistance. Various seed layers, multiple underlayers and multi-layered magnetic films exist in the prior art. The multi-layered magnetic films include multiple ferromagnetic layers that are ferromagnetically coupled and more recently antiferromagnetic coupling has been proposed. Seed layers may be used with nonmetallic substrate materials such as glass. Typically the seed layer is a relatively thin layer that is the first crystalline film deposited in the structure and is followed by the underlayer. Materials proposed for use as seed layers include chromium, titanium, tantalum, MgO, tungsten, CrTi, FeAl, NiAl and RuAl.
Prior Art FIG. 1B illustrates layer structure 21 of a thin film magnetic disk 16 according to conventional art. The layers under the underlayer 33 may be any of several combinations of seed layers 32 and pre-seed layers 31 as noted in more detail below. One example includes a pre-seed layer of CrTi50, a seed layer of RuAl (B2 crystal structure) and a CrTi10 underlayer. The substrate 26 can be any prior art substrate material, either glass or metal. The magnetic layer stack 34 can be composed of a plurality of layers that are further illustrated in Prior Art FIG. 1C.
Prior Art FIG. 1C is an illustration of a conventional magnetic layer stack 34 having at least four distinct layers. The bottom magnetic layer 44 is a ferromagnetic material of the type used in the prior art of thin film disks. Examples of materials suitable for bottom magnetic layer 44 might include CoCr, CoPtCr and CoPtCrB. The coupling/spacer layer 43 is typically a nonmagnetic material with a thickness that is selected to antiferromagnetically couple the top magnetic layer structure 40 with the bottom magnetic layer 44.
The top magnetic layer structure 40, according to the prior art, can be a bilayer structure including two distinct ferromagnetic materials. The interface (first) sublayer 41 is a thin layer of material with a relatively high moment, that is, a moment higher than the second sublayer 42. The preferred materials for the interface sublayer 41 are CoCr, CoCrB and CoPtCrB. The interface sublayer 41 material is selected to have a higher magnetic moment than that the second sublayer 42. Some prior art layer structures, such as layer structure 40, have a third, middle sublayer. In all known examples having a middle sublayer, the middle sublayer includes a Pt component.
Referring to Prior Art FIG. 1D, an illustration of typical grain structure in a section of a magnetic layer (e.g., sublayer 40 of top magnetic layer structure 40) is presented. These grains, such as grain 110, are small columnar grains. The columnar granular structure goes all the way through the layer. When a transition is written, it follows along the rows of grains. To a first order, the number of grains that exist in a bit affects the signal-to-noise ratio (SNR). In an ideal situation in which all grains (for example, grain 110) are identical in shape, size, and properties, the SNR is proportional to the square root of the number of grains in a bit. However, in reality, the grains vary in properties, such as size, as is illustrated by the difference between diameter 140 and diameter 150. With different size grains, the distances from one to another can vary, as illustrated by distance 120 and distance 130. Other properties of the grains such as their magnetic moment and internal anisotropy can also vary which will lower the media's SNR.
Historically, to improve SNR, the grains of the media have been made smaller. As the grains become smaller and smaller, their energy becomes comparable to thermal energy and the material begins to lose stability (super-paramagnetic limit). This situation is being realized and, thus, the historic pathway to improving SNR in thin film media is coming to an end.